The present invention relates generally to relay station transponders and more particularly to orbital relay station transponder, such as are used in geosynchronous communication satellites, that selectively provides for both global and spot beam antenna coverage.
Conventional satellite transponders have been developed that provide for global and spot beam coverage. One conventional transponder with a selective antenna beam comprises a set of transponder channels that are respectively coupled to two multiplexers. Each multiplexer feeds its own antenna network which each comprises an array of power couplers, phase shifters and feed elements. A selectable channel is separately coupled by way of a variable power divider to the respective multiplexers and hence to the two antenna networks. The selectable channels can selectably power one of the antenna arrays to provide a spot beam or both of the arrays to provide a zonal beam. Reference is made to an article by Robert F. Buntschuh entitled "First Generation RCA Direct Broadcast Satellites," IEEE Journal on Selected Areas in Communications, Vol. SAC-3, No. 1, Jan. 1985., for example, which describes such a conventional antenna beam transponder.
Alternately, in a second conventional transponder, a set of transponder channels are coupled by way of a multiplexer to an antenna network comprising a set of hybrid couplers, phase shifters and feed elements that produce global beam coverage. A selected number of the transponder channels have transfer switches that couple power to a separate single antenna network to provide spot beam coverage. Reference is made to an article by M. D. Harwood et al., entitled "The Aussat Spacecraft Communications Payload: An Overview," IREECON '83, Sydney, Australia, Sept. 1983, for example, which describes such a conventional antenna beam transponder.
These conventional transponders utilize either two full multiplexers in the first case, or a second multiplexer for the switched channels and two separate antennas in the second case to provide for the distinct zonal and spot beam coverage. Such transponder designs are complex, costly to manufacture and have excessive weight which in many instances cannot be tolerated, such as in a spacecraft application where weight is a major concern.
Accordingly, it would be desirable to have a transponder that minimizes the number of components required to provide both global and spot beam coverage, has less weight, and which is less costly to manufacture.